Can't open with for flatpak apps (sandbox issue)


I would like to starting using flatpak apps, but cannot figure out how to allow one flatpak app to see another one.

My use case: Currently I use repo versions (I’m on Linux Mint) of GIMP, RawTherapee and Shotwell. In Shotwell and Digikam I can set up the applications to ‘open with’ another app. For example, if I want to edit an image in GIMP, or edit a RAW image in Rawtherapee. However, when I install the flatpak versions of those apps, I cannot choose to open with another flatpak app. I understand the whole sandbox concept. I installed flatseal and added the paths to the flatpak folders (somewhat shooting in the dark). This still does not allow me to see other flatpak apps.

  1. What am I doing wrong?
  2. I think there should be some system level flatpak customization options to allow apps to see other apps - that don’t require one to install flatseal and go hunting for some random documentation on a forum somewhere. At a basic level if one trusts an app, one should be able to allowed to let that app be accessed from other apps with the same OS (providing the user allows it).